Datasheet

9
9596A–AT42–10/10
AT42QT1070
2. Overview
2.1 Introduction
The AT42QT1070 (QT1070) is a digital burst mode charge-transfer (QT
™
) capacitive sensor
driver. The device can sense from one to seven keys, dependent on mode.
The QT1070 includes all signal processing functions necessary to provide stable sensing under
a wide variety of changing conditions, and the outputs are fully debounced. Only a few external
parts are required for operation and no external Cs capacitors are required.
The QT1070 modulates its bursts in a spread-spectrum fashion in order to heavily suppress the
effects of external noise, and to suppress RF emissions. The QT1070 use a dual-pulse method
of acquisition. This provides greater noise immunity and eliminates the need for external
sampling capacitors, allowing touch sensing using a single pin.
2.2 Modes
2.2.1 Comms Mode
The QT1070 can operate in comms mode where a host can communicate with the device via an
I
2
C-compatible bus. This allows the user to configure settings for Threshold, Adjacent Key
Suppression
®
(AKS
®
), Detect Integrator, Low Power (LP) Mode, Guard Channel and Max Time
On for keys.
2.2.2 Standalone Mode
The QT1070 can operate in a standalone mode where an I
2
C-compatible interface is not
required. To enter standalone mode, connect the Mode pin to Vdd before powering up the
QT1070.
In standalone mode, the start-up values are hard coded in firmware and cannot be changed.
The default start-up values are used. This means that key detection is reported via their
respective IOs. The Guard channel feature is automatically implemented on key 0 in standalone
mode. This means that this channel gets priority over all other keys going into touch.
2.3 Keys
Dependent on mode, the QT1070 can have a minimum of one key and a maximum of seven
keys. These can be constructed in different shapes and sizes. See “Features” on page 1 for the
recommended dimensions.
• Comms mode – 1 to 7 keys (or 1 to 6 keys plus Guard Channel)
• Standalone mode – 1 to 4 keys plus a Guard Channel
Unused keys should be disabled by setting the averaging factor to zero (see Section 5.9 on
page 21).
The status register can be read to determine the touch status of the corresponding key. It is
recommended using the open-drain CHANGE
line to detect when a change of status has
occurred.
2.4 Input/Output (IO) Lines
There are no IO lines in comms mode.
In Standalone mode pins OUT0 – OUT4 can be used as open drain outputs for driving LEDs.